Heme proteins iron active site

The protein from D. desulfuricans has been characterized by Mbss-bauer and EPR spectroscopy 224). The enzyme has a molecular mass of approximately 150 kDa (three different subunits 88, 29, and 16 kDa) and contains three different types of redox-active centers four c-type hemes, nonheme iron arranged as two [4Fe-4S] centers, and a molybdopterin site (Mo-bound to two MGD). Selenium was also chemically detected. The enzyme specific activity is 78 units per mg of protein. 

Achieving fast electron transfer to enzyme active sites need not be complicated. As mentioned above, many redox enzymes incorporate a relay of electron transfer centers that facilitate fast electron transfer between the protein surface and the buried active site. These may be iron-sulfur clusters, heme porphyrin centers, or mononuclear... 

The active-site model (and the ONIOM model system) includes Fe, one aspartate and two histidine ligands, a water ligand and selected parts of the substrate (see Figure 2-6). The 2-histidine-1-carboxylate ligand theme is shared by several other non-heme iron enzymes [59], For the protein system, we used two different... 

The state of knowledge on iron-sulfur proteins which contain non-heme iron bonded to sulfur ligands (cysteinyl residues from the protein and inorganic sulfur) has been reviewed by several authors258"264. With magnetic resonance techniques it has been possible to obtain detailed information on the nature of the active site in many of these proteins. The contributions from ENDOR have recently been summarized by Sands265 so that we shall only give an outline of the crucial points. 

The interaction within the active site can be either in the form of covalent binding or in the form of quasi-irreversible (tight but slowly reversible) binding, and it can involve the protein residues, the porphyrin moiety or the catalytic center (heme iron) [8]. CYP inactivation follows a stoichiometry of one substrate molecule per enzyme molecule inactivated. To measure the stoichiometry of the inactivation, it is necessary to trap all molecules that are not specifically bound to the active site, by using an appropriate scavenger, normally GSH. 

The enzyme catalyzing the formation of retinal 2 by means of central cleavage of P-carotene 1 has been known to exist in many tissues for quite some time. Only recently, however, the active protein was identified in chicken intestinal mucosa (3) following an improvement of a novel isolation and purification protocol and was cloned in Escherichia coli and BHK cells (4,5). Iron was identified as the only metal ion associated with the (overexpressed) protein in a 1 1 stoichiometry and since a chromophore is absent in the protein heme coordination and/or iron complexation by tyrosine can be excluded. The structure of the catalytic center remains to be elucidated by X-ray crystallography but from the information available it was predicted that the active site contains a mononuclear iron complex presumably consisting of histidine residues. This suggestion has been confirmed by... 

The coordination sphere around the ferrous iron center is almost octahedral with the heteroscorpionate clamp causing a deviation of 4-6° from the ideal 90° angle. A comparison with very high-resolution protein structures such as IPNS or CAS clearly shows a good correspondence of structure 4b in distances and angles with the active sites of the non-heme iron enz5unes (Table I). 

Perhaps the most fundamental fimctional property of a heme prosthetic group at the active site of a heme protein is the relative stability of the reduced and oxidized states of the heme iron. A number of structural characteristics of the heme binding environment provided by the apo-protein have been identified as contributing to the regulation of this equilibrium and have been reviewed elsewhere 82-84). Although a comprehensive discussion of these factors is not possible in the space available here, they can be summarized briefly. The two most significant influences of the reduction potential of the heme iron appear to be the dielectric constant of the heme environment 81, 83) and the chemical... 

Active Site Structure of Rubredoxin There are several non-heme iron-sulphur proteins that are involved in electron transfer. They contain distinct iron-sulphur clusters composed of iron atoms, sulphydryl groups from cysteine residues and inorganic or labile sulphur atoms or sulphide ions. The labile sulphur is readily removed by washing with acid. The cysteine moieties are incorporated within the protein chain and are thus not labile. The simplest type of cluster is bacteria rubredoxin, (Cys-S)4 Fe (often abbreviated FelSO where S stands for inorganic sulphur), and contains only non labile sulphur. It is a bacterial protein of uncertain function with a molecular weight of 6000. The single iron atom is at the centre of a tetrahedron of four cysteine ligands (Fig.). 

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